One type of tandem color laser printer well known in the art includes four photosensitive drums and developing rollers for the colors yellow, magenta, cyan, and black. In this type of tandem color laser printer, the developing rollers form toner images in each respective color on the photosensitive drums at substantially the same time. Subsequently, the toner images are transferred onto paper or an intermediate transfer belt, enabling color images to be formed at approximately the same speed as an image formed on a monochromatic laser printer.
One such tandem color laser printer proposed in U.S. Patent Application Publication No. 2004/165910 (corresponding to Japanese Patent Application Publication No. 2004-279551) includes developer cartridges each with developing rollers, and drum units each having a photosensitive drum such that the developing cartridges and drum units can be individually mounted in or removed from a main casing of the laser printer.
Another image-forming device disclosed in U.S. Pat. No. 6,738,590 (corresponding to Japanese Patent Application Publication No. 2003-50531) has an image-forming unit in which are integrally assembled a plurality of image-bearing members, a plurality of developer units, a plurality of charging units, and at least one intermediate transfer member. This integrated assembly can be mounted in or removed from a main body of the image-forming device.